Recently, along with progress in digital signal compression technology of moving picture or integrated circuit for digital signal processing, in the television broadcast by broadcast satellite or communication satellite, the analog transmission system is being replaced by the digital transmission system which is capable of presenting broadcast and communication services of higher picture quality in multiple channels, and its spread in the general household is highly expected. In this background, there is an increasing demand not only for smaller size and lower cost of low-noise converter used in the receiving antenna of broadcast or communication satellite radio waves, but also for further improvement of phase noise characteristics of the microwave oscillator incorporated in the low-noise converter which has a serious effect on the quality of bit error rate (BER) of decoded signals in the receiver when the quadrature phase shift keying or octantal phase shift keying is employed as the digital transmission system.
FIG. 6 is a circuit diagram of a microwave oscillator hitherto used as the local oscillator of a low-noise converter. At base terminal of a bipolar transistor 1 for microwave oscillation (hereinafter the bipolar transistor is called merely as a transistor be distinguished from the field effect transistor or FET), a microstrip line (MSL) 4a is connected. In this line 4a, a choke circuit composed of high impedance line (HIL) 6a and MSL 4d, and a dc voltage divider composed of dc resistances 3c and 3d are connected serially as shown in the diagram, and one end of the resistance 3c is connected to a bias supply terminal 2b of the transistor, and a bias voltage is applied to the base terminal therefrom. A capacitor 7c for grounding the microwave component leaking out from the HIL 6a is connected parallel to the resistance 3d, and one end is grounded. At a terminal end of the MSL 4a, a series circuit of a high frequency resistance 3b and MSL 4c of which one end is released, having a length of λg/4 is connected as shown in the diagram, and a resistive terminal end is formed in the MSL 4a. Herein, λg is the guide wavelength of the MSL 4c corresponding to the oscillation frequency. (Hereinafter, λg is meant as the guide wavelength determined by the corresponding MSL. The value of λg varies with the line width of MSL, thickness of circuit board, and dielectric constant.)
The MSL 4b is connected to the collector of transistor 1, and in this line 4b, a choke circuit of HIL 6b and MSL 4f and a dc resistance 3a are connected as shown in the diagram, and one end of the resistance 3a is connected to a bias supply terminal 2a of the transistor, and a bias voltage is applied to the collector terminal therefrom. A capacitor 7b for grounding the microwave component leaking out from the HIL 6b is connected serially to the resistance 3a, and one end is grounded.
A hollow circular dielectric resonator (DR) 5 couples the MSL 4a and 4b electromagnetically, and is arranged so that the distance from its release end to the closest position to the center of the DR 5 may be λg/4 on the MSL 4b. 
In this configuration, by feeding back from the collector to the base of the transistor 1 through the DR 5, the oscillation frequency and oscillation power are stabilized at the frequency determined by the resonance frequency of the DR 5, and the oscillation power is delivered from an output terminal 8 through an impedance matching circuit 9 and a dc blocking capacitor 7a connected to the emitter of the transistor 1.
Meanwhile, the HIL 6c and MSL 4e connected to the matching circuit 9 form a choke circuit, and are used for preventing leak of microwave oscillation power and grounding the emitter of the transistor 1 in direct current.
This conventional configuration, however, requires choke circuits composed of HIL 6a, 6b, 6c and MSL 4d, 4f, 4e which are connected for applying a dc bias to the MSL 4a, 4b connected to the corresponding terminals of the transistor 1 and connected to the impedance matching circuit 9, and these choke circuits make it difficult to reduce the size of the microwave oscillator or induce deterioration of phase noise characteristics, generation of spurious oscillation, or other unstable oscillation.